The present invention relates to the use of the drug valproic acid and derivatives thereof as inhibitors of enzymes having histone deacetylase activity. The invention also relates to the use of those compounds for the manufacture of a medicament for the treatment of diseases which are associated with hypoacetylation of histones or in which induction of hyperacetylation has a beneficial effect for example by induction of differentiation and/or apoptosis in transformed cells.